Overvoltage protection devices having thermal disconnection apparatus have been known for years in the prior art. The function of a disconnection apparatus is, in the event of unacceptably high heating of the overvoltage protection unit, e.g. a varistor, to safely disconnect it from the electrical network before the arrestor suffers any damage.
Disconnection apparatus comprising solid sliders are known from EP 0 987 803 B1 and EP 0 905 839 B1. The respective slider is spring-loaded and each protect a varistor from unacceptable thermal heating. The closed position of the respective connection elements is fused by a solder. The solder is subjected to the constant spring preload force conveyed by the slider, which is a substantial disadvantage. Depending on the rating; i.e. the desired current-carrying capacity, a plurality of disconnection apparatus are required, which constitutes a disadvantage in terms of costs. It has furthermore been shown with the above prior art solutions that solder particles or particulate matter become entrained and form solder filaments during the disconnection process and the displacing motion of the slider, resulting in insufficient electrical disconnection.
The DE 295 19 313 U1 utility model indicates an overvoltage protection device having a thermal disconnection apparatus, wherein the disconnection apparatus is positioned separately from the actual varistor by means of a partition wall. The surge current carrying capacity of such a disconnection apparatus is too low from today's perspective and its solder joint subject to a constant spring preload force.
The known DE 2 220 264 A relates to a thermal protection or warning device in which actuation is blocked by a temperature-sensitive retaining element and the retaining element activates a slider as a result of melting upon a predetermined temperature being reached.
DE 1 515 019 A discloses a temperature limiter in which two leaf springs are soldered together by solder which melts upon a certain temperature being reached and are used particularly in fan heaters.
The generic DE 10 2007 042 991 B4 solves the task of specifying a further developed overvoltage protection device having a mechanical disconnection apparatus activated in the event of thermal overload, wherein the solder joint of the connection elements which releases upon disconnection is not to be constantly subjected to the effect of a mechanical force load. The known solution moreover exhibits a high surge current carrying capacity, wherein the surge current itself produces the contact pressure for the discharging process. The disconnection apparatus according to DE 10 2007 042 991 B4 is realized such that the feed to the voltage-limiting overvoltage protection units, particularly varistors, is formed by two conducting sections extending parallel in the end region and merging into a common solder joint fused by solder with surge current flowing through the sections in the same direction. Upon surge current, the respective sections of the leads, respectively brackets, attract the parallel current flow and the thereby associated effects of the force and at that time effect the desired increase in contact force on the solder joint. This makes it possible to dimension the solder joint solely to fix the sections of the brackets, which enables the mechanical dimensioning of the solder joint to be limited to an absolute minimum and thereby have it be particularly thermally sensitive. The spring of this respective disconnection apparatus, the function of which consists of displacing a solder between the current paths when the solder joint melts and thus interrupt the electric circuit, can be realized with lower spring force so as to result in lower constant loading of the solder joint. Since the slider drives directly into the solder joint upon disconnection, negative effects are prevented during the disconnection process and safe, voltage-free cut-off achieved.
Disadvantageous with respect to the DE 10 2007 042 991 B4 solution is the fact that the slider is functionally coupled with the visual signaling of the slider position and thereby a display of the functional properties of the disconnection apparatus. For this reason, the slider thus needs to be structurally designed so as to exhibit the most even, color-differentiated display area possible in order to make clear the respective functional state of the respective overvoltage protection device. Furthermore, the slider's direction of movement upon overload for disconnecting the varistors preferably utilized is predefined so that the position of a display window for identifying the state of movement of the slider is also structurally determined, which represents a disadvantage for example when the size of the respective overvoltage protection device is decreased.